Light gas detector using thin walled glass tubes for diffusion



1963 K. B. MGAFEE, JR 3, 68

I LIGHT DE TECTOR USING THIN WALLED ASS TUBES FOR DIFFUSION Filed Aug. 20, v1958 2 Sheets-Sheet 1 DIFFUS/ON lNl/EA/TOR A. 8. Mc AFEE, JR.

A ORNE V AFEE, JR 3,100,868

K. B. M LIGHT GAS DETECTOR USING THIN WALLED GLASS TUBES FOR DIFFUSION Aug. 13, 1963 Filed Aug. 20, 1958 2 Sheets-Sheet 2 F/G. 2 PL or CF PERMEAB/L IT) ors/L/cA GLASS r0 VARIOUS GASES He //V S/L ICA I727 727 393 227 I27 60 I3 23 TEMPERATURE c.

)X/l/EA/fOR K. 8. Mc AFEE, JR.

A TTORNEV mass spectrometer.

United States Patent "LIGHT GAS DETECTOR UING THIN WALLED GLASS TUBES FOR DIFFUSIGN Kenneth 18. Median, in, Summit, N.J., assign'or. to Beli Telephone Laboratories, incorporated, New York, NJEL, a corporation of New York Filed Aug. 20, 1958, SeruNo. 756,115 r 6 (Jlaims. (Ci. 324-33) The invention relates to a process and apparatus for the detection of gases. l

Large pipelines are today carrying vast quantities of hydrogen and helium. Laboratories and industries are using increasing amounts of the tracer gases such as hydrogen, helium, deuterium, tritium and neon. Vessels that are to be pressurized are tested with tracer gases to insure against leaks. As a result, there is an increasing demand for a simple, inexpensive and portable gas detector that is highly sensitive'to the tracer gases enumerperiods of time withoutdeterioration,

The detectionv of gases by this device is dependent upon the selectivity of thinwalled glass tubing tothe enumerated gases. The sensitivity of' the device to varying amounts of one or all of the' gases is proportional to the surface area of'the' tubing which is increased by using a multiplicity of tubing The tubing is arranged so as to be contacted on the outside by a gas mixture containing one or more of the enumerated gases. One end of the tubing is closed while the other end opens into a common enclosure. The actual detection of the gas or gases that diffuse through the tubing Walls is achievedbya is connected to the common receptacle.

A more complete understanding of the features of this invention together with additional objects thereof may be gained from the following description in conjunction with the accompanying drawings in which: 7

FIG. 1 is a front elevational view partially in section showing at detecting element in conjunction with a sensing element; and

FIG. 2 on coordinates of permeability. versus temperature is a plot of permeability of a preferred glass material to various gases.

With reference now more specifically to the drawings, the device shown in FIG. 1 is thirty-six inches long and utilizes a sensing element comprising a gas-tight metal jacket 1 formed for example of Kovar metal having gas inlet 2, gas outlet 3, and an insulated heating Wire 4 encircling the surface. One end of the jacket 1 is closed by metal cap 5. The jacket 1 encloses approximately two miles of silica tubing 6 having closed ends 7 and open ends 8. The tubes have outside diameter of seventeen mils detecting element such as unionization manometer which T e device is also sensitive to tracer gases suchflas hydro- .gen, deuterium, tritium and neon. The ;devic'e-is rugged, inexpensive, portable and can be'keptin' storage for long and la wall'thick ness of three mils. The closed ends of the tubing are sealed in a gas-tight floating head 9 of, for example, epoxy resin arranged to keep the tubes in a fixed position. The open ends of the tubing pass through a gas-tight seal It of, for example, epoxy resin which provides a gas-tight barrier between-enclosure 11 and enclosure 12 with the latter enclosure common to the open ends h of tubes '6; Neck 13 is connected to an ionization manometerwhich comprises a housing 14 having outlet 15 provided with valve 16 leading to a diiiusion pump, not shown. Housingld encloses filament 17 having ten volts impressed thereon, anode 18 havingplus 150 volts impressed thereon, and cathode 19 having minus fifteen volts impressed thereon.

In operation, the detector which operates at room pressure is placed in the vicinity of a leak. The gas escaping through the leak goes into inlet 2 of the detector, diffuses through the tubing 6 and hence into the detecting element which is maintained at a partial vacuum of 10" millimeters of mercury. The gas atoms flowing into the detecting element are ionized by electrons from filament J 17. The resulting ions flow to cathode 19 andinduce a current thereon. current, which is Ia measure of the amount of gas diffusing through the tubing can be measured by a galvanometer, for example by a Veeco Vacuum Gage, which amplifies the current before measurement. Since under normal operating conditions the tubing walls are virtually impermeable to atmospheric gases, the enre measured induced current is due to the leak. If there is a gas in the atmosphere to which the detector is sensitive, thegalvanometer can be adjusted to give a compensate-d zero reading before being placednext to the leak. j

Although specificvalues have been used in describing the device of FIG. 1,it is to be understood that these values are-exemplary only. I i

The detector is designed to operate at room pressure with a partial vacuum; for example, of' 1(l to 10' millimeters of mercury maintained inside the tubing and detecting unit. The amount of gas per secondildifiuising through the tubing is a functionof the'diameter of the" tubing to the wall thickness. A ratio of from four to one to six'to one islpreferred. The time'for diffusion is desirably, small, in the orderof one second or less.

time is proportional to the wall thickness squared over the diffusion constant. 7 l V T ubing having an outside diameter ranging from one 1 mil to fifty mils and a wall thickness varying from 0.2

mil to ten mils is suitable These limits are based on practical considerations. Thinner tubing is generally undesirable only from the standpoint of fragility; thicker tubing although operative necessitates additional bulk and affects 7 only portability.

In general it is preferable to form the tubes of a glass which includes at least a total aggregate of seventy-five percent of one or more of the glass-forming substances [silica (SiO boron oxide (B 0 aluminum oxide (A1 0 and phosphorus trioxide (P 0 Glasses oomprising at least seventy-five percent of silica and glasses comprising at least seventy-five percent of silica and a maximum of fifteen percent boron oxide (B 0 are among the better glasses for the purposes of this invention. For example, fused silica glass silica), chemical Pyrex glass [81% silica (SiO 13% boron ox ide (B 0 2% aluminum oxide (Al O hand 4% sodium oxide (Na O) and/or potassium oxide (K OH and Referring to FIG. 2, the eifect of temperature on the permeability of silica tubing to various gases is plotted. At room temperature exceptionally pure helium is diffused through the silica tubing even though the initial gas mixture: may contain hydrogen and neon. This results since helium with its atoms only 2A. in diameter diffuses through the walls or" the tubing, whereas hydrogen, the

next smallest atom, While being only twenty-five percent larger than helium, has a diffusion constant through the tubing one thousand timessmaller. Larger gas molecules such as methane with a diameter of 2.5 A. has such small diifusion constants that they will not-pass through the glass tubing'in any significant quantity.

As theoperating temperature is raised, for example, by Nichrome fWl-ffi i surroundingmellic jacket 1, kinetic energy is imparted to gas molecules surrounding the ceramic tubing 6. This energy increases the diffusion con- At approxi-j stant of each of the gases in the mixture. rnately 165 C. hydrogen commences to diffuse through the tubing in appreciable amounts. 'At an even more elevated temperature, approximately 270 C, neon commences to diffuse through the tubing in appreciable amounts. The temperature range of this, device may vary from room temperature to 400 for Pyrex tubing, and

' from room temperature'to 800 C. for silica tubing With- Percent helium Original End pressure Diflerence pressure (mm. 'Hg) 1. 3. 4= 10 5. 2 10,- 1. 22 1()- 0. 3.8 10- 4. l 10 8 80x10- 0. 5.8 10- 8. 0 10- 5 20 10 O. 3.6)(10- 4.1X10- 1 18 10 0.0004 4.4x10- 4 73x10- What is claimed is: V

1. A gas detecting device comprising a sensing element having a multiplicity of thin walled glass tubes having 'an outside diameter varying from. one mil to fifty mils and a Wall thickness varying from 0.2 mil to ten mils through which a gas to be detected diffuses and a detecting element sensitive to the gas 'diifusing through the glass tubes.

2. The detecting device in accordance with claim 1 wherein the tubes are made of a glass having a silica content of at least seventy-five percent.

3. The detecting device in accordance with claim '1 wherein the tubes are made of a glass comprising a minimum of seventy-five percent silica (SiO and a maximum 'of fifteen percent boron oxide (B 0 4. The detecting device in accordance with claim'l wherein the tubes are made of a glass comprising at least a total aggregate of seventy-five percent of one or more of the glass forming substances selected from the group consisting of silica (SiOg), boron oxide (B 0 aluminum oxide (A1 0 and phosphorus trioxide (P 0 5. The detecting device-in accordance with claim 1 wherein means is provided for maintaining a partial I vacuum of 10* to 10" millimeters of mercury;

out any deteriorating effects on the tubing. It is evident I that by using two or more sensing elements operating at selected temperatures in combination with a detecting element, itis possible to detect any one gas of the group consisting-of helium, hydrogen, deuterium, tritium, or neon from a gas mixture. 7

As an alternative arrangement, the gas mixture may 1 be admitted to theinside of the glass tubing. :The gas to'be detected then diffuses out'ot the tubing and news intoi the detecting elementthis invention using Pyrex tubing to varying percentages of helium present in a gas mixture. The operating temperature-was maintained at 357 C. to 392 C. and a I The detector of the present invention capable of detecting leaks of very small'magnitude. For. example,

the following table shows the sensitivity of a detector of i In this example,the induced current was read as a function-of the difference multiplied by a factor of 23.6 between the end pressure reading and the original pressure reading on the manometer.

6. A gas detecting device comprising a gas-tight jacket having a gas inlet and a gas outlet, a heating wire encircling said jacket, a multiplicity of thin walled glass tubes having an outside diameter varying from one mil to fifty mils and a Wall thickness varying from 0.2'mil to ten mils enclosed by said jacket, said .tubes'being sealed at one end and opening into a common enclosure at the other end, said enclosure being connected with 'an ionization manometer, and means for countingthe ions pro-:

'iduced by said manometer. 1

References 'Citediin' theifileof thisi patent V f 5 N S ATESPAIENI McCollum 2,526,038 Oct. 17, 1950 2,671,337 Hulsberg Mar. 9, 1954 2,734,592 1 Jones Feb. 14, 1956 2,909,919 Myer Oct. 27, 1959 2,921,210 Schaschlet a.. Ian. 12, 1960 g FOREIGN PATENTS 7 1,507 Germany July 8,1926

May 28, 1-946 

1. A GAS DETECTING DEVICE COMPRISING A SENSING ELEMENT HAVING A MULTIPLICITY OF THIN WALLED GLASS TUBES HAVING AN OUTSIDE DIAMETER VARYING FROM ONE MIL TO FIFTY MILS AND A WALL THICKNESS VARYING FROM 0.2 MIL TO TEN MILS THROUGH WHICH A GAS TO BE DETECTED DIFFUSES AND A DETECTING ELEMENT SENSITIVE TO THE GAS DIFFUSING THROUGH THE GLASS TUBES. 